Blade tracking mechanism for lifting rotors



Dec. 9, w52 H. T. AVERY 2,620,883

' BLADE TRACKING MECHANISM FOR LIFTING ROTORS Filed MaICh lo, 1947 2SHEETS-SHEET l BY @at 4 fw 4free/Veys.

H. T. AVERY BLADE TRACKING MECHANISM FOR LIF-T ING KOTORS -2sx-mETs--srmn'r 2 Filed March 10, 1947 INVENTOR. /zfwaw IAA/52V BY@7L/.4.4

Patented Dec. 9, 1952 BLADE TRACKING MECHANSD'I FOR LIFTING RO-'IORSHarold T.Avery, Oakland, Calif.

Application. March 10, 1947,'Senzial4 No. 733,673

Claims.

The present invention relates t'o sustaining rotors for aircraft, andparticularly to rotors of the articulated type, that isl to rotorscomprising blades each attached to a central hub by one or more hingesincluding a flapping hinge permitting of vertical angular displacementof the blade relative lto the hub.

In such a rotor each blade is, at. each instant, angularly positionedabout its flapping hinge in accordance with the resultant of' all forcesacting on the blade at the instant, including the effect of the momentumand inertia of the blade itself. It is the usual practice yin Asuch,rotors, particularly in helicopters, to provide some means for effectingcyclic control of pitch so that the blades may be Vcaused to vfollowsome non-symmetrical path or to oiTSet the nen-symmetrical effect on theblades of forward flight or other translational movements of the craft.

Unless each blade is an exact duplica-te of each other blade bothmechanically and in aerodynamic shape it will tend to seek a ldifferentangular position and to track higher -or lower under at least someconditions of flight. For instance, if one blade has a slight ltwisttending to give it a slightly higher pitch setting near the tip thanthat lof the other blades it will tend to track higher than the otherblades. It is customary to provide some means whereby the pitch settingmay be decreased on a blade which is thus found to be tracking too high,but since such means normally serves to reduce pitch over the entireblade the best that it can do is to permit of setting .the blade so thatthe pitch over the inner portion of the blade (that is the portioncloser 'to the hub) will be enough less than that of .the other bladesto counterbalance under certain operating conditions the remainingexcess of pitch setting in the outer portion of the blade. n this mannerthe blade may be made to track properly on .the test stand where theadjustments are made, `but under certain *flight conditions wherein theproportion of the blade lift contributed by the outer portion of theblade differs considerably from that pertaining under the testconditions the blade will again fail to track properly with the otherblades,

The immense variety in the nature and location of the `difference inaerodynamic shape that may occur between blades, together with the eX-and aerodynamic forces set up by discrepancies if' Cl. 17d-1.60.13 )4 inblade tracking. Present methodsof retrackingl blades are howeververyslow, awkward and expensive, and though they are capable ofproviding good tracking `und-er` certain chosen sets of operatingconditions., they will not in general, prevent the lblades from goingout of proper tracking and causing vibration under other sets ofoperating conditions. Y

Itis an object of this invention `to cause the various blades, of arotor to automatically seek the same path `or track in spite ofdifferences in construction vand/or aerodynamic Sharpe of the respectiveblades.

It is a further object to provide such automatic 'tracking continuouslyduring flight and in spite of changes in the operating conditions.

It is a further object to equalize the average effective flappingpositions of all blades at each point in the circuit, taking into'account displacement due to exing of the blades as well as those due toangular displacementv of the blades on their flapping hinges.

lit is an 'object to provide Such retracking in a manner that will notaffect the average collective pitch of all blades.

It is a further object to control the rate of retracking so that on theone hand it will be sufficiently great so that tracking errors will beminimized substantially as rapidly as they 'Will occur, and yet on theother hand retraeking will not occur so rapidly as to produce adverseetfects upon the cyclic control of the rotor.

It is also an object to vary the rate of retracking in accordance withthe amount of the difference in blade positions, so that large trackingerrors can be corrected at rates which, if brought into playb-yslight'cyclic control displacements of the blades, would undulydisturb such slight control displacements.

It is a further object to restrict such variation in the rate ofretracking to substantial-ly the maximum range of diierences in bladepositions that may be related to tracking errors, so that the rateofretracking will increase vwith increase in size of the tracking error upto the maximum tracking errors that may be anticipated, but will notcontinue to increase throughout all of the much greater range ofdifferences in blade positions which occur on account vof cyclic controldisplacements within which `greater range further increase in the .rateof retraeking would serve no useful purpose but would unnecessarilyincrease te@ adverse selects-er cycli@ @Qn-trol- The manner in which theforegoing, together with aslitorel @bg-legis an@ advantages Vf the Rotorconstruction and drive The invention is illustrated in Figures 1 and 2as applied to a three-bladed rotor constructed for the most part,substantially as disclosed in my co-pending United States PatentApplication, Serial Number 665,653, filed April 29, 1946, now Patent No.2,539,562, dated January 30, 1951.

As particularly shown in Figure l each blade (the three such bladesbeing identified in Figure 2 as Illa, IIlb, and IUc respectively)comprises a skin or covering integrally mounted on ribs |2 which in turnare integrally attached to the tubular blade spar I3, which spar ter- Lminates inwardly in a bearing retainer |4 containing a ball thrustbearing I5 co-axial with the adjacent portion of the spar. This bearingserves to attach the blade to the connecting link I6 in a mannerpermitting of the blade being rotated about the spar axis relative tothe link I6, to effect changes in the pitch setting of the blade.Connecting link I6 is in turn attached by means of flapping hinge |1 tolugs I8 integral with hub member I9. Hub member turn pivotally mountedby means of roller bearings and 2|, for rotation about cylindricalmember 22 and about the co-axial cylindrical member 23 which is attachedto member 22 by means of a plurality of bolts 24, which bolts togetherwith their co-operating nuts 3|) serve to attach both members 22 and 23to the cylindrical member 3| which is integrally attached to ring 32 andfuselage frame members 33.

Attached to the bottom of hub member I3 by means of a plurality of bolts34 is a ring 35 having downwardly extending lugs 36 for receiving theroller bearing 20. The spherical roller thrust bearing 31 is interposedbetween ring 35 and cylindrical member 23, thus serving to transmit tothe framework of the craft the upward thrust of hub member I9, which isprimarily the force which sustains the craft in fiight. Attached to thetop of hub member I9 by means of a plurality of bolts 38 (Fig. 2)

is a plate 39. Interposed between this plate and a flange 40 (Fig. 1) ofcylindrical frame member 23 is ball thrust bearing 4|, which serves tosustain the rotor when it is not exerting an upward lift on the craft.

The drive for the rotor cornes from the engine, not shown, throughtransmission shaft 42, which shaft is guided in the upper surface 44 ofthe cylindrical frame member 22. Integral with the upper end of shaft 42is gear 50 which meshes with idler 5|, which idler is rotatably mountedon stud 52 which is integrally mounted in member 22. Idler 5I in turnmeshes with teeth 53 cut into the inner face of hub member I9. Hubmember |9 is thus rotated upon the fixed cylindrical member 22, 23 bythe rotation of shaft 42 by the engine.

Collective and cyclic pitch setting As previously described, blade i0 isattached to connecting link I6 in a manner permitting of |9 is in 2.

the blade being rotated about the blade axis relative to link I6, toeffect changes in the pitch setting of the blade. Blade I0 is rotatablypositioned about this blade axis by the vertical posi- `tioning of theblade pitch controlling means which comprises link 6| connected by aball and socket joint to arm (Fig. 2) integral with the blade. Thelength of link 6| is rendered adjustable by constructing it of twosections screwed together and locked by nut 62 (Fig. l).

The vertical positioning of pitch control link 6| is effected by a pitchcontrol spider 65 comprising arms 64 integral with ring 65a, each link6| being connected by means of a ball and socket joint to a bell-crank63 pivotally mounted on a corresponding one of these arms 64. Each pitchcontrol bell-crank 63 is maintained in a fixed position relative to itsrespective arm 64 except to the extent that it may be displaced relativethereto by the automatic blade tracking mechanism, which is describedhereinafter. Pitch control spider 65 is rotated about pitch control ring66 on ball bearings 61 in unison with the rotor, the force for rotationbeing supplied by pin 68 which is integrally mounted on hub member I9,and protrudes through a slot in arm 39 integral with pitch controlspider 65.

Pitch control ring 66 is integrally attached to pitch control hub 10 bymeans of arms 1| which arms include necked down round portions 12 whichpass through cylindrical member 3| in slots 13. Pitch control hub 10 isattached to collective pitch control rod 14 by ball and socket joint 15.One cyclic pitch control assembly is illustrated in Figure l, comprisingcyclic pitch control arm integrally attached to pitch control ring 66and attached to cyclic pitch control rod 8| by ball and socket joint 82.There is also another cyclic pitch control assembly (not shown) whichcomprises another cyclic Ditch control arm 80 and rod 8| like those justdescribed, but this assembly is attached to ring 66 at a locationdisplaced ninety degrees from that of the assembly illustrated. Thecollective pitch control rod 14 and the cyclic pitch control rods 8| areconnected by a linkage such that vertical movement of the collectivepitch control rod 14 will be accompanied by the same amount of verticalmovement of both of the cyclic pitch control rods 8|, but the cyclicpitch control rods 8| are each capable of independent vertical movementrelative to collective pitch control rod 14. Such a type of linkage isshown on pages 122-130 of the June 1945 issue of Aviation magazine, anda different form of such a linkage is shown in my co-pending UnitedStates Patent Application, Serial 630,745, led November 26, 1945, whichissued as Patent No. 2,546,881 on March 21, 1951.

The operation of the pitch control assembly is as follows: The upwardmovement of collective pitch control rod 14 (automatically accompaniedby similar upward movement of both rods 8|) raises ring 66 withoutaltering its angle of tilt, thereby simultaneously increasing the pitchon all of the rotor blades |0 by means of the linkages previouslydescribed. The upward or downward movement of one of the two cyclicpitch control rods 8| relative to collective pitch control rod 14inclines ring 66 and hence pitch control spider 65 relative to the rotorhub axis at an angle dependent upon the position of the cyclic pitchcontrol rod 8| relative to rod 14. The inclination of pitch controlspider 65 causes the rotor blades to increase and decrease their pitchactricesL cyclical-ly; i. e., the pitch is increased by that 'segmentlof the pitch control lspider that 'is raised and conversely the pitchis decreased by that segment of the pitch control spider that is loweredon account of the inclination of the pitch control spider. Thus spider65 and the mechanisxn which controls it, as above described consti'tutesthe primary pitch adjusting means fcr the rotor, but its action issubject to alteration by the secondary pitch adjusting means hereinafterdescribed.

Mechanism responsive to blade flemme Because of the fact that rotorblades are ordinarily constructed of relatively slight verticaldimension and sustain relatively great aerodynamic loads of uneven andcyclically changing distribution along the length of the blade, suchblades are ordinarily subjected to considerable flexing in flight,particularly in a vertical direction. If a blade which is attached tothe rotor hub by a flapping hinge is appreciably exed, the effectiveflapping position of the blade, and therefore its tracking lof the otherblades, is dependcnt upon the direction and `amount of the ilexingoi theblade as Well as upon the vertical angular displacement of the rootsection of the blade about the flapping hinge. In order to take both ofthese factors into account I provide mechanism which is quantitativelyresponsive to blade iiexure as well as to displacement of the bladeabout the napping hinge. This mechanism includes a lever 91 (Fig. l)which is angularly positioned about napping hinge Il in response to bothvertical flexing of the blade and angular displacement of the blade onnapping hinge This angular positioning of lever 91 is effected by thefollowing mechanism:

Lugs 90 are integrally attached to tubular blade spar I3. Flexiblecables or wires r| are attached to these lugs by means or" pins 92 andare led along paths vertically displaced from the neutral axis of theblade through guides 53, each of which guides may consist of a pair ofrollers mounted on a lug integral with spar i3. Near the root of theblade each cable 9| is attached to a connecting link 911i by means of aconnecting pin 95. Connecting links Qd are pivotally attached by meansof pins 96 to lever 97|. Pins dii are placed considerably further apartthan are the pins 92 at the outer ends of cables 9|, so that the angulardisplacement oi lever di will be correspondingly less than that of lugsThe reason for this is so that lever 9i will be angularly displaced inproportion to the average eiTective angular displacement or" the bladerelative to the hub due to blade flexing, as measured at the nappinghinge. This angular displacement is substantially that of a lineconnecting the flapping hinge to a point on the blade somewhere in thevicinity of the average effective center of lift of the blade, and is inany case very considerably less in amount than the angle through whichthe outer tip portion of the blade has been ilexed. Therefore, if lugs90 are located rather close to the blade tip, pins Sli should be aboutthree times as far apart as pins 92 so that lever el will be rockedthrough about one-third the angle through which lugs 9c are deflected,while if lugs 90 are located further in on the blade the ratio of thedistance between pins 95 to that between pins $22 should becorrespondingly decreased, so that in any case, whether the spacing isdetermined by theory or by test, the rflexing of the blade in flightshall displace lever 91 'through an angle equal to that 6 through whichthe unilexed blade would have to be rocked .around flapping hinge inorder to produce substantially the same amount of change of bladetracking eiiect on the craft.

These same connections to lever 91 will also cause that lever to rockabout napping hinge in unison with the blade, .for any given conditionof blade exure. Hence lever 91 is angularly positioned about hinge inaccordance with the average effective blade position taking into accountboth blade iiexure and rocking ofthe blade upon the flapping hinge.

Blad/e tracking mechanism As previously indicated it is highly desirableto eliminate all differences in blade tracking under all conditions ofoperation and mechanism is therefore provided which is sensitive to anydifferences that may exist between the angular position of the variouslevers 97 associated with the respective blades, and which isautomatically operable to decrease such differences by adjustment of thepitch of one 0r the other or of both of the blades with reference towhich the difference exists. In the preferred form of blade trackingmechanism herein described pitch is automatically adjusted on both ofthe blades concerned.

This blade tracking mechanism consists of two major parts: the firstpart constitutes a tracking control means comprising a valve systemtogether with the linkages which supply that system With indications ofthe differences in blade tracking, and the second part constitutes anoperating means comprising a hydraulic system that alters blade pitch inaccordance with the responses of the valve system, in a manner adaptedto eliminate the differences in blade tracking.

The valve system and its linkages are as follows: In conjunction witheach blade there Vis provided a slide |00 which moves in a guide I0! andis connected to the corresponding lever Si (previously described) by aslide extension H32, connecting link |03, and pins |04 and |05. Verticalpin |06 is integrally attached to slide |00, and ts in the fork of onearm of a bell-crank |01 (see also Fig. 2) which is pivotally mounted onpin |08 which in turn is integrally mounted in hub member IS. The otherarm of the bellcrank is pivotally attached by means of a pin ||0 to alink |09, Which link is made adjustable in length by constructing it oftwo pieces clamped together by means oi bolts l2 extending through slotsEach link |09, in addition to being connected at one end to a bell-crank01, as previously described, is pivotally connected at the other end toone end of a lever ||3 by means of a pin I4. 'At its opposite end leverI3 is provided with a fork which spans the one of the previouslymentioned vertical pins Ii which is positioned` by the next adjacentblade. Near its mid-point lever IIS is pivotally attached to avalve-positioning link H5 by means of a pin H5. Link I5 thus constitutesan element jointly positioned by two of the blades in accordance withthe difference of the eiiective napping angle of the two blades. Eachlink ||5 is normally held in fixed relation to a corresponding slidevalve member I1 by means which nevertheless permit of its beingyieldably displaced relative thereto in either direction. This meanscomprises a spring l I8 which yslips over tenons Hd, which areidentically shaped on both link H5 and member Il?, as parts of identicalH-shaped openings in the two members. Each member H5 lies directly ontop of the corresponding member ||1 and in the plan view, as illustratedin Figure 2, the portions of these two members extending outwardly frompin I6 are identical. This construction is generally similar to that ofthe spring link illustrated in Figure 50 of Avery Patent No. 2,271,240dated January 12, 1942. However, each member H terminates inwardly withan arcuate surface concentric with the corresponding pin ||6, as shownin dotted lines in Figure 2, while the corresponding slide Valve member||1 extends on inwardly and includes as an integral part thereof a slidevalve |20, the three such valves shown in Figure 2 being identified as|20a, |2013, and |20c respectively. Each slide valve |20 slides in valvesleeve |2| (the respective sleeves being identified as |2la, |2|b, and|2|c) and its movement therein is limited by stop lugs |22 integral withvalve |20. The purposes of the Valve override spring ||8 is to permitmovement of valvepositioning link ||5, in either direction after slidevalve |20 and the integral extension ||1 thereof may have reached thelimit of their travel and are prevented from moving by stop lugs |22.

It is to be understood that, as illustrated in Figure 2, the abovedescribed valve mechanism is repeated three times for a three-bladedrotor, and that each valve is associated with two adjacent blades. In amanner hereinafter set forth the valve mechanism just described operatesto displace one of the three slide valves |20 from neutral wheneverthere is a difference in effective flapping angle between the two bladeswith which said slide valve is associated.

Each slide valve |20 thus constitutes an adjustable element positionedby the blades for controlling the blade tracking operation, and, in themanner hereinafter described, these valves act as the controllingmembers for the pitch adjusting or altering portion of the bladetracking means.

As previously mentioned the second part of the blade tracking mechanismconsists of the hydraulic system which constitutes the operating meansfor altering blade pitch to reposition the blades so as to maintainproper tracking. Hydraulic pressure and hydraulic flow for all threeblades is supplied by the single hydraulic pump |30 which is attached toplate 39 by means of a plurality of bolts |3|, and plate 39, aspreviously described, is attached to hub member |9 by means y of aplurality of bolts 38. Hydraulic pump |30 is equipped with aconventional spring loaded bypass |32 which is used to permit all excesshydraulic fluid pumped to flow back to intake while maintaining constanthydraulic pressure on the output side of the pump. Hydraulic pump |30 isoperated by shaft |33 (Fig. 1) which is turned by spur gear |34 which isintegrally attached to shaft |33. Spur gear |34 is rotated by idler gear|35 which turns on stud |36 which is integrally attached to plate 39.Idler gear |35 is, in turn, rotated by idler gear |31 which is free torotate on stud |38 which is integrally attached to plate 39. Idler gear|31 is rotated whenever the rotor is turning by virtue of the fact thatit meshes with gear teeth |39 cut on the inside face of flange 40 ofcylindrical frame member 23.

The output side |40 (Fig. 2) of hydraulic pump |30 is connected to thethree valve intake ports ,|4|, and the intake side |42 of the pump isconnected to the three valve exhaust ports |43. Each valve |20, |2| isassociated with two of the three downpass tubes |44 and serves toselectively connect the intake of pump |30 to one or the other of thesetwo downnass tubes and to connect the dynamic forces.

output of the pump to the other of'said tubes |44. As indicated inFigure 1 each of these tubes |44 includes a flexible looped portion|44a, through which it is connected to a hydraulic cylinder |45, thethree such cylinders being labelled |45a, |451), and |45c, respectively,in Figure 2. These cylinders constitute operating units peculiar to eachblade. Each such hydraulic cylinder |45 contains a piston |41 backed upby a spring |46 against which the hydraulic pressure must press.Integrally connected to each piston |41 is a piston rod |48 which has aright angle bend at its outer extremity, this bent portion passingthrough slot |49 in pitch control bell-crank E3. Each piston |41constitutes a pitch adjusting unit of the previously mentioned secondarypitch adjusting means, for displacement of a piston |41 will rock theassociated bell-crank 63 and through it raise or lower the correspondingpitch control rod 6| to adjust the pitch setting of the associated oneof the blades I0. The pumping of fluid into a cylinder |45 forces itspiston |41 and piston rod |48 outward, which through bellcrank 63 forcespitch control rod 6| upward which increases the pitch of thecorresponding blade and causes the blade to rise due to aero- Thereforeincrease in the volume of fluid in a cylinder |45 raises the respectiveblade and conversely a decrease in such volume lowers the blade.

Slide valve |20a (Fig. 2) is positioned in response to differences inthe effective flapping angle of blades |0a and |0b and in turn regulatesthe flow to and from the hydraulic cylinders |45a and |45b which adjustthe pitch of these two blades, while slide-valve |20b is similarlypositioned by differences between blades |0b and |0c and similarlyadjusts the pitch of these two blades to eliminate such differences, andslide-valve |200 is similarly positioned by differences between blades|0c and |0a and adjusts the pitch of these two blades to eliminate suchdifferences.

Operation of the blade tracking mechanism is as follows: For the purposeof illustration assume that blades |012 and |0c are tracking, but thatblade |0a is travelling higher than the other two. This will cause thelever 91 which is associated with blade |0a to rotate slightlycounterclockwise (as viewed in Fig. l) about flapping hinge |1. Thiswill cause vertical pin |06 on the corresponding slide |00 to moveinward, carrying inward the lower end (as viewed in Fig. 2) of the lever||3 which is associated with slide-valve |20a. This will causeslide-valve 20a to move inward in valve-sleeve |2|ia. The inwardmovement of said vertical pin |06 will however also causecounterclockwise rocking of the bell-crank |01 with which it isassociated, thus through link |09 causing outward movement of the rightend (as viewed in Fig. 2) of the lever |3 associated with slide-valve|20c, thus causing said slide-valve to move outward in valve sleeve |2|c. The inward movement of slide-valve |20a will cause hydraulic fluidto be pumped from the cylinder |45a, which is associated with blade |0a,and into the cylinder |45b, which is associated with blade |011. Thiswill lower the pitch on blade Illa and raise the pitch on blade |0b,thus causing decrease in the effective flapping angle of blade |0a andincrease in that of blade |0b. The outward movement of slide-valve |20cwill cause hydraulic fluid to be pumped from the cylinder |45a and intothe cylinder |45c, which is associated with blade |0c. This will lowerstill further the pitch on blade |0a and raise the pitch on blade |0cthus further 9 lowering the path of blade Illa and raising that of blade|00. Thus blade Illa will be lowered at twice the rate. that blades ||lband ||lc are raised. This process continues until all of the 4blades aretracking again, whereupon the centering of valves ld terminates theprocess.

Thus if any blade stands at a flapping angle different from the mean ofthe napping angles of all blades it will be caused to change itsflapping angle inthe direction of that mean at a rate substantiallyproportional to the amount of its departure from such mean Value.

In view of the fact that the above described mechanism is operated insuch a mannerr as to eliminate differences in eiective flappingpositions of the blades it would', if it operated rapidly enough,seriously interfere with the cyclic differences in flapping position,which constitute the accepted method of exercising control of the craftthrough an articulated rotor. To avoid this, either the openings |53(Fig- 2*) in all of the valves |2il, or the'xedparts with which theycooperate, or both, are made of very slight dimension in thedirectionperpendicular to the plane of Figure 2 andV they thereforeserve to markedly throttle the rate of fluidflow, therebyserving as ratecontrol means for the operating means. As shown in Figure 2 theseopenings |50 are of considerable width in the plane of Figure 2'.v Bythus providing openings |50 which are of considerable width in thedirection of travel of valves |26 (which is in the plane of Figure 2)-and of very slight eiiective dimension in the direction perpendicular tothe plane of Figure'Z, themarked throttlin-g effect provided by valves|253 is selectively altered by displacement of a valve |20 so that theow through the valve opening is approximately proportional to thedisplacement of the valve from the position at which it completely cutsoli the flow. This fact, combined with the fact that the Valvedisplacement is arranged to normally be proportional to the differencein eifective flapping angle, regulates the flow to an amount which is atleast roughly proportional to such dierence, thus providing forcorrection of the majority of each tracking error in approximately aconstant period of time regardless of the amount. of such error. Withsuch an arrangement the functioning ofv this mechanism does notappreciably affect the amplitude of cyclicchanges in blade position dueto the fact that such changes normally involve equal amounts of movementoi each blade above and below its mean napping position and therefore inpassing from its upper to its lower extreme,y position or vice versa itwill be subject to two. substantially equal and opposite increments ofcorrection by the blade tracking mechanism, which increments willtherefore substantially balance' each other. The action of the bladetracking` mechanism will have a tendency to advance all cyclic changesin blade path by a small and` substantially constant and predictableazimuth angle, which therefore. can be allowed for in orienting thecyclic control mechanism. It is therefore, practicable to construct thevalves with openings su-iiiciently large to correct, for instance,approximately one-half of any given tracking error within a single-cycleof rotor rotation.

By constructing valves |211A so-as to provide. a

proportionately regulated throttlingpeiect on the hydraulic iiow theopposing requirements for promptly eliminating tracking errors and yetknot disturbing cyclic. blade; differences: are reconciled. Considerablelatitude is possibleliin the degree of 10 such throttlingV that willgive satisfactory results but there are rather` sharply dened limits tosuch latitude. For instance if the throttling limits the flow to a rateproportional' to the dif-` ferences in effective blade positions, andifr the vertical dimension of openings |50 is such that the iiow duringone cycle of normal rotor operation will eliminate half the diiierencein blade positions, then it will require 4.3 cycles to eliminate 95% of`the error or about onek and a fraction seconds, which is fully as shorta time as willv be required for introducing anyv marked change in flightconditions. Therefore, the response is rapid enough toA providesubstantially instantaneous correction oftracking errors.

The amountof correction that would be introduced in one quarter cycleisindicative of' the amount of'efiect' on cyclicqcontrol, and' thisshould be a small fraction of the total error, not more than one-thirdin any case and preferably less. Any more rapid introduction ofcorrection than this would commence to introduce secondaryvirregularities of objectionable size, due' principally to the fact thatthe previously mentioned' tendency of the blade tracking'corrections tobalance out i-n each half cycle of rotor operation is not perfectlyattained if the amount off cyclic tilt of the rotor is not constantoverthev halfcyc'le. For instance if the-amount, of cyclictilt is 5%greater or less during the lasthalf' of such a half cycle period than itis during the" iirst half of it, and if the correction effective yin aquarter cycle is 0.33 of the difference existing then the'secondaryirregularity introduced would' be ofthe amount of cyclic differenceV indflapping position of two adjacent-blades. This-fact limits the rate atw-h-ich correction' should be introduced to not more than 0.801of thetracking error'in one cycle, for this rate corresponds to correction of0.33 of the tracking error in a quarter cycle, which as above indicatedis substantially the maximum that can be tolerated. At this rate 95% ofthe tracking error would be eliminated'in 1.86 cycles, which isextremely prompt: correction.

|The lower limi-t on rate` of correction is, im.- posed by the factthatv retracking should take place substantially as* rapidly as flightconditions can change. On this basisv not over 4 seconds should beallowed for 95% correction, which at 200 R. P. M. represents 13.3cycles, which corresponds to 0.20of the tracking error corrected in onecycle. At this rate only 0.054V of' the error is corrected inone-quarter cycle, which would reduce the secondary irregularitiesabovementioned to one-sixth of the limiting values described.

Hence satisfactory operation will be secured if the rate of correctionis held substantially within the range of 20% to 80% of the errorcorrected in one cycle of normal rotor operation, but` departure much ifany beyond this range oommences toY introduce either too slow a responseor too marked'irregularities upon rapid change of cyclic tilt.

.As previously indicated the rate of correction maybe established at anyselected value byappropria-tely choosingtheV vertical dimension of valveopenings |50. In order to secure the; co1'- rection of a substantiallyconstant fraction of any tracking error duringl each cycleit would benecessary for the remainder of the hydraulic system to. be lofsu-iiciently large 'cross-section to produce negligible friction loss.However, the permissible latitude in retracking rates is' so 11 greatthat very considerable friction losses can be tolerated, so long as theyare not so great as to cause the resultant retracking rate to fallsubstantially outside the limits hereinabove indicated as acceptable forblade differences within the range of such differences that may be dueto tracking errors. For diierences in blade positions greater than thosewhich may be due to tracking errors, and such greater diiferences willordinarily occur cyclically in connection with rotor tilt, it will notonly be permissible for the tracking correction per cycle to become lessthan 0.20 of the difference, but highly desirable for it to do so forlarge cyclic differences If the valve area is so related to thecharacteristics of the remainder of the hydraulic system that the amountof valve opening corresponding to the maximum difference in bladeposition caused by tracking errors permits almost as much hydraulic flowas the amount of flow that would pass through the system with the valveentirely removed, then the maximum rate of retracking for any amount ofblade difference will be only slightly greater than that required forproper correction of maximum tracking errors. However, to providegreater latitude in the design of the system, and particularly in orderto permit of faster correction of maximum tracking errors combined withsharper limitation on the maximum rate of retracking I prefer to providethe limit stops |22, previously described, arranged to limit themovement of valve |20 in each direction, and hence prevent any furtherincrease in the rate of retracking, after the valve has been displacedfrom neutral by at least the distance corresponding to the maximum bladedifference due to tracking error. In order that tracking errors combinedwith large cyclic blade differences may produce selective positioning ofthe valves adapted to reduce the tracking errors throughout aconsiderable part of the cycle, and in view of the fact that suchselective positioning ceases while the valve is held against its limitstop, I prefer to place the limit stops so that they do not becomeeffective until the valve has been displaced considerably beyond theposition corresponding to the maximum tracking error itself, and to havethe friction losses in the system such that they gradually reduce thepercentage of error corrected per cycle for valve displacementsintermediate between those corresponding to maximum tracking error andthose at which the stops |22 are encountered.

With the blade tracking mechanism constructed in the particular mannerwhich has been illustrated and above described, a difference ineffective flapping angle of any two blades automatically initiates areadjustment of pitch on both of said blades in a manner tending toeliminate the difference. It is not essential, however, that pitch bereadjusted on both blades for the difference could be eliminated byapplying the correction to either of the two blades. For instance thevalve |200 is positioned in accordance wlth the difference between thenapping angles of blades Illa and |c. Through two openings in the valvesleeve |2|c it is connected to tube |5| which connects it to the pitchadjusting mechanism of blade |Ua, which is the leading one of these twoblades, while through two other openings in the valve sleeve |2|c it isconnected to tube |52 which connects it to the pitch adjusting mechanismof blade Ic, which is the following one of these two blades. Aspreviously described, if blade Illa is positioned at a higher flappingangle than blade Ille, hydraulic fluid 12 pumped through tube |5|operates to decrease the pitch on blade la, and that pumped through tube|52 operates to increase the pitch on blade |00, until the diierence inflapping angle is eliminated. However, if tube |5| and its openingsthrough sleeve |2|c were removed the difference would still beeliminated, for the iluid pumped through tube |52 would still operate toincrease the pitch on blade lOc until the difference disappears. Thus ifthe three tubes which lead forward from the three valves and correspondto tube |5| were thus eliminated, or flow through them was permanentlycut off, the blade tracking system would operate by bringing each bladeto automatically track the blade ahead of it. If instead of eliminatingthe tubes corresponding to tube |5| the three corresponding to tube |52were eliminated or plugged, the blade tracking system would operate bybringing each blade to automatically track the blade next behind it.However, by including vboth sets of tubes appreciably prompter and moresatisfactory tracking can be secured. For instance if as previouslyassumed for illustration blades |0b and |60 are tracking but blade |0ais travelling higher, then with tubes |5| eliminated valve |20a acts tolower blade Illav to eliminate the difference between blades lila andIb, while valve |20c acts to raise blade Ic to eliminate the differencebetween blades |0a and |Dc. Since there is initially no differencebetween blades |0b and IOc, valve |20b does not initially alter thepitch of blade Ilb, but as blade |00 is raised a difference between thepositions of blades |01) and |0c develops and serves to raise blade lbat a rate proportional to the amount it remains below blade |0c whichwould be at a slower rate than if it were also controlled by thedifference between blades |0a and |017, as it will be in case all thetubes illustrated in Figure 2 are functioning.

What I claim is:

1. In an aircraft having a sustaining rotor including a hub and aplurality of blades each attached to said hub by means permitting ofchange in the angle subtended between the blade axis and the rotor axisand means permitting of change in the pitch angle', and primary pitchadjusting means connected to each of said blades; blade tracking meansindependent of said primary pitch adjusting means and comprisingsecondary pitch adjusting means, pitch controlling means for each ofsaid blades comprising a member jointly positioned by said primary andsaid secondary pitch adjusting means, a member connecting each of saidpitch controlling means to its respective blade so as to angularlyposition the blade in pitch, rate control means independent of saidprimary pitch adjusting means for controlling the rate of operation ofsaid secondary pitch adjusting means, and including an adjustableelement and connections from said blades to said adjustable element toposition said adjustable element in accordance with differences in theangles subtended between the respective blades and the rotor axis.

2. The invention set forth in claim l in which the secondary pitchadjusting means comprises a plurality of pitch adjusting units eachconnected to a respective one of said blades to control the pitch anglethereof; said rate control means comprises a plurality of rate controlunits each including an element selectively positionable to control therate of operation of one of said pitch adjusting units; andsaidconnections include a plurality of members each connected to one' of"said elements and toja plurality of blades including the' blade' towhich the respective' pitch adjusting unit is related. t

3. The* invention set forth in claim l, `in which said blades 'are'flexible', in combination with a member in' conjunction with each blade;Yeach of said members Vbeing mounted on the hub'for movement relativethereto' in response to vertical ilexing ofthe blade, a part integrallyattached to theb'lade' inthe' outer half of the' length there` of, and aflexible link connected to said part and to said member and extendinginward along the blade and guided'relative thereto so' as to impartAmovement to saidv member upon vertical flexing of said' blade, and inwhich said positioningl of said adjustable element of. said rate controlmeans by said blades is eiiected through said members. l l

4. The invention set forth' in claim l, in which said means permittingof change in the anglesubtended between the blade axis andthe rotor`axis comprises a flapping hinge and a. flexible root portion of saidblade, in combination with-a member in conjunction with each blade, eachof said members being mounted on the hub for movement relative theretoin joint response to`V the flexing of the blade andthe displacement ofthe blade upon' its hinge relative to the hub, a part integrallyattached to the bla'dein theouter-hali thereof and a flexible linkconnected to said part and to said member and extending inward along theblade and guided relative thereto so as to impart movement tosaidmernber upon vertical'flexing and/or'displacement of 'said bladerelative to said hub, a connectionr from said flexible link to saidmember along a line removed from the axis of said flapping hinge, andinA which said'positioning of said adjustable element of said ratecontrol means by said bladesis eiiected' through said members.

5. In an aircraft having a sustaining rotor including a hub and aplurality of blades each attached to said hub by means permitting olchange inthe flapping angle and'l by means permitting or change in thepitch angle of the blade; pitch adjusting means for each of said bladescomprising a, cyclic pitch control member, manually positionable meansmechanicallj7 control'- ling the position of said member, apitchadjusting member., means for automatically positioning said memberunder joint control of two of said blades in accordance with therespective napping 'anglesV of said two blades,Y a pitch setting'mem'-ber connected to said blade at a point remote from the blade axis so asto determinev the angular position of said blade about said axis, andpositioning means for said pitch setting member jointly positioned bysaid cyclic pitch control member and said pitch 4adjusting member andconsisting of mechanical linkage extending .from said cyclic pitchcontrol member and said pitch adjusting member to said pitch settingmember.

6,. The invention set forth inwclaim 5 wherein the means forautomatically positioningthe vpitch radjusting member comprises 'ratecontrol means selectively settable to corresjxondinglyV determine therate o such positioning, an element, connections 'from said element to'each of two' of said blades r'to position said element in accordancewith the difference between the effective ilapping angles of said twoblades, and means connecting said element to said rate control means toset said rate control means in accordance with the position of saidelement.

7. The invention set forth in claim 6 in com- 14 bination with a surfaceconnected to saidrate control means and la `cooperating surface fixed inthe path or movement of said rst mentioned surface, so as to'block saidsurf-ace and thereby limit the range of positions to which said ratecontroll means may be set by said element.

8; in anairc'raft having a sustaining rotor including a hub and aplurality offlexibleblades attached to said hub bymeans including apitch changing hinge constituting a pivotal support having an axislongitudinal of the blade; blade tracking-means including a pitchadjusting memberrconnected to one of said blades at -a point remotefromv said Ypitch changing hinge so-asto selectively alter the pitchthereof in laccordance with the displacement of said pitch adjustingmember, means for automatically displacing said pitch adjusting memberunder joint control of two ofA said blades in accordance with thedifierence in the vertical-flexural positionl ofsaid two bladesincluding two parts each associated with a respective one of said twoblades and integrally attached Ato the blade in the outer half lof thelength thereof, two flexible links each associated with a respective oneof said two blades and connected to the respective' one of said twoparts and extending inward along the blade and guided relative theretoalong a path vertically displaced from the neutral axis or the blade, alever connected to each of said two links and jointly positioned bythem, operating means for applying power from said source tosaid pitchadjusting member to displace same, adjusting means forselectivelyadjusting said operating means tocontrolY the direction andvrate at which said power is supplied, and a connection from said leverto said adjusting means to adjust` said operating means in jointresponse to the ilexing of `saidtwo blades.

t 9. Inl `an aircraft having aV sustaining rotor including a hub and aplurality of ilexibleblades each attached to said hub by meanscompri-sing a flapping hinge and a pivotal mounting substa-ntiallyparallel to the axis of the blade;-blade tracking means including apitch adjusting member connected to one of said blades at a poi-ntremote from said pivotal mounting so as to angularly displace said bladevon Vits respective pivotal mounting in accorda-ncefwi-th thedisplacement of said pitch adjusting member, means for automaticallydisplacing said pitch adjusting member under join-t `control oftvv'o-ofsaid blades in accordance with the difference inf'their effectivevertical positions as determined -bytheir respective angulardisplacements about their respective flapping hinges plus theirrespective verlticalflexura-l displacement, including two parts each1associated with a respectiveonefof said two blades and integrallyattached itc, ther-'blade in vtheeu'ter half 1of the length thereof,ytwo-ficxible -linksfeac'h attached toa respective 4one of said partsuand extending 'inw-ard along the blade and-guided'relative "theretoalong a 'rpath verticallyY displaced from the neutral axis-of the blade,a lever, la connection from-each of said links tosaid lever, each of'said connections being vertically displaced-'from the respectivenapping hinges, `a source of power, operating means for uapplyinglpower'fronrsaid source 'to said pitch adjusting member tri/displacesame, Vadjusting"means for selectively adjusting said operating 'meansto control the direction and rate at which said power is supplied, yanda connection from said lever to said adjusting means to adjust saidoperating means in joint response to the flexing of said two blades.

10. In an aircraft having a sustaining rotor including a hub and aplurality of blades each attached to said hub by means permitting ofchange in the flapping angle of the blade and means permitting of changein the pitch angle of the blade; blade tracking means comprising ahydraulic pump, a hydraulic cylinder having a piston, a hydraulicconnection from the pump to the cylinder to operate the piston, a valveinterposed in said connection between said pump and said cylinder forcontrolling the operation of said piston by said pump, linkageconnecting said valve to two of said blades to position said valve inaccordance with the difference of the flapping angles of said twoblades, a, manually positionable pitch controlling element, a pitchcontrolling member connected to said element and said piston so as to bejointly positioned by said element and by said piston, and mechanicallinkage connecting said pitch controlling member to one of said twoblades to establish the pitch angle thereof.

11. The invention set forth in claim 10 in in which said valve isprovided with ports which are of slight dimension in the directionperpendicular to the travel of the valve and to the direction of flow ofthe fluid through the valve as compared to their dimension in thedirection of travel of the valve.

12. In an aircraft having a sustaining rotor including a hub anda'plurality of blades each attached to said hub by means permitting ofchange in the flapping angle of the blade and a pivotal mountingsubstantially parallel to the axis of the blade; blade tracking meansincluding a plurality of pitch adjusting members each connected to arespective one of said blades so as to angularly position said blade onits respective pivotal mounting, a plurality of controlling membersmounted on the hub for individual displacement relative thereto, linkageconnecting each of said blades to two adjacent ones of said controllingmembers and thereby connecting each controlling member to two adjacentblades to automatically displace said controlling member relative tosaid hub in accordance with the difference between the flapping anglesof said two blades, an operating connection to each pitch adjustingmember from the two controlling members which are connected to the bladeto which said pitch adjusting member is connected for displacing saidpitch adjusting member.

13. In an aircraft having a frame and a sustaining rotor including a hubrotatable with respect to said frame and a. plurality of blades eachattached to said hub by means permitting changes in the flapping angleand pitch thereof; pitch changing mechanisms comprising a pitch controlspider tiltably supported with respect to its center, a plurality ofbell-crank levers, a fulcrum on said spider for each of said bell-cranklevers, each fulcrum mounting its respective lever for rocking movementin a vertical plane and means connecting an arm of each of said leverswith one of said blades for changing the pitch of said connected bladeupon displacement of said fulcrum in a vertical plane or upon rocking ofsaid spider upon its fulcrum; separate primary and secondary adjustingmeans connected with said pitch changing mechanism, said primaryadjusting means comprising a train of mechanism 16 carried by said frameand connected to said spider to tilt same; and said secondary adjustingmeans comprising a plurality of hydraulic cylinders mounted on saidspider, a, piston in each of said cylinders connected with a second armof a respective one of said bell-crank levers; and a control system forsaid secondary adjusting means comprising a source of fluid pressure,

a connection between each of said cylinders and said source, valve meansincluding an adjustable element interposed in each of said connections,a lever fulcrumed on each of said adjustable elements, means including aconnection between one of said blades and a point on one of said leversspaced from the fulcrum thereof for adjusting said element in onedirection upon upward flapping movement of one of said blades, and meansincluding a connection between another of said blades and a point on oneof said levers oppositely spaced from the fulcrum thereof for adjustingsaid element in the opposite direction upon upward flapping movement ofsaid other of said blades.

14. The invention set forth in claim 13 in which said valve meansincludes a throttling passage partially opened in selective amounts bythe displacement of said adjustable element whereby the amount of fluidsupplied to said cylinders by said source of fluid pressure isproportioned to the difference between the flapping angles of ytheblades connected to said element.

l5. In an aircraft having a frame and a sustaining rotor including a hubrotatable with respect to said frame and a plurality of blades eachattached to said hub by means permitting changes in the flapping angleand pitch thereof; pitch changing mechanisms comprising a pitch controlspider, a plurality of devices mounted on said spider and movable eitherby said spider or relatively thereto in a substantially verticaldirection, and means connecting each of said devices with one of saidblades for changing the pitch of the connected blade upon either suchmovement of said device; separate primary and secondary adjusting meansconnected with said pitch changing mechanisms, said primary adjustingmeans comprising manually operable means for moving said spider and thesaid devices mounted thereon in a substantially vertical direction, andsaid secondary adjusting means comprising means carried by said hub andresponsive to said blades upon changes in the flapping angles thereoffor moving said devices relatively to said spider in a substantiallyvertical direction.

HAROLD T. AVERY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,800,470 Oehmichen Apr. 14, 19312,356,692 Platt Aug. 22, 1944 2,397,154 Platt Mar. 26, 1946 2,397,489Jenkins Apr. 2, 1946 2,408,489 Stalker Oct. 1, 1946 2,439,089 HodsonApr. 6, 1948 2,444,070 Stanley June 29, 1948

